For an optical semiconductor encapsulating resin composition for use in the production of optical semiconductor devices such as light emitting diodes (LEDs), a transparent epoxy resin composition that is obtained using an epoxy resin, such as a bisphenol A epoxy resin and an alicyclic epoxy resin, in combination with an acid anhydride as a curing agent is generally used.
In optical semiconductor devices such as LEDs in which an epoxy resin composition is used as an encapsulating resin, internal stress is generated by the contraction that occurs during the curing of the epoxy resin composition and by the distortion that results from the difference between the linear thermal expansion coefficients of the epoxy resin and an optical semiconductor (such as an (LED chip), lead frame or the like. Moreover, long-term use of an LED results in gradual property deterioration due to the change of its environmental conditions, causing problems such as the breaking of the cured product of the epoxy resin used as an encapsulating agent and the reduction of the luminance thereof. Such deterioration is evaluated in terms of heat cycle resistance, which indicates the durability of LEDs in a heat shock test.
For a method to reduce the internal stress of an LED, a method in which inorganic powders such as silica powder are added to an epoxy resin composition to reduce the linear thermal expansion coefficient of the composition to be close to that of a LED chip or the like is performed. This method, however, is problematic in that, although internal stress is reduced, transparency is impaired because large amounts of inorganic powders are required to adjust the linear thermal expansion coefficient to be close to that of an LED chip or the like.
In order to address the above-described problem, Patent Literature 1 proposes a method for enhancing the transparency of an epoxy resin composition by adding silica powder that is surface-treated with a silane coupling agent to minimize the difference between the refractive indices of the silica powder and the cured product of the epoxy resin composition. In addition, Patent Literature 2 discloses a method for enhancing the transparency of an epoxy resin composition by adding silica particles having a particle diameter of 0.06 μm or less, which is shorter than the wavelength of light. Furthermore, Patent Literature 3 discloses a method for enhancing the transparency and the heat resistance by treating at high temperatures silica particles that are added to an epoxy resin composition or surface-treating them with hexamethyldisilazane or a silane coupling agent to decrease the hydroxyl groups on the particle surface and to adjust the particle diameter thereof to 0.1 μm or less.
The aforementioned methods are, however, all problematic in that the transparency of a cured product is still insufficient and the cured product readily undergoes cracking during a heat shock test, exhibiting inferior heat cycle resistance, because the dispersion stability of silica powder in the epoxy resin composition is poor and silica particles that are in an agglomerated state are present.
Therefore, a great demand exists for an epoxy resin composition for encapsulating an optical semiconductor, the cured product of which has high transparency, i.e., high light transmittance, and excellent heat cycle resistance.
Patent Literature 1: JP 11-74424A
Patent Literature 2: JP 06-302726 A
Patent Literature 3: Japanese Patent No. 3017888